Disk drives are widely used in computers, consumer electronics and data processing systems for storing information in digital form. The disk drive typically includes one or more storage disks, and one or more head suspension assemblies. Each head suspension assembly includes a slider having an air bearing surface, and a read/write head that transfers information to and from the storage disk. The rotation of the storage disk causes the slider to ride on an air bearing so that the read/write head is at a distance from the storage disk that is referred to as a “head-to-disk spacing” (also sometimes referred to herein as a “flying height”).
Because today's disk drives utilize storage disks having increasingly high densities of data tracks, decreasing the head-to-disk spacing has become of great importance. However, this desire for a very small head-to-disk spacing must be balanced with tribological concerns in order to avoid damage to the read/write head and/or the storage disk, as well as loss of data.
Further, a large variation in the head-to-disk spacing from slider to slider can cause significant issues in the manufacturing and reliability of the disk drives. Additionally, maintaining a relatively small and consistent head-to-disk spacing is further complicated by other factors such as thermal pole tip protrusion caused by thermal expansion of the read/write head (also referred to herein as write pole tip protrusion or “WPTP”), or by an overall temperature increase of the disk drive (also referred to herein as environmental pole tip protrusion or “EPTP”) during various drive operations. For example, during a write operation, the electrical resistance of the write element generates heat in and around the read/write head, resulting in thermal expansion of a portion of the slider toward the storage disk. If the pole tip protrusion is too extensive, the slider can unintentionally contact the storage disk, causing off-track writing, damage to the slider, damage to the storage disk and/or a permanent loss of data.
Thus, knowing the actual in-situ head-to-disk spacing at various times during operation of the disk drive can be critical. Unfortunately, the ability to accurately determine the actual head-to-disk spacing in-situ has been extremely elusive. In conventional disk drives, only a determination of relative changes in head-to-disk spacing, i.e., an increase of 2 nanometers, a decrease of 3 nanometers, etc., has been attained without knowing the actual in-situ distance between the read/write head and the storage disk during operation of the disk drive.